1. Field of the Invention
The present invention generally relates to a high frequency signal measuring equipment and, more particularly, to a high frequency signal measuring equipment having a cabled detector.
2. Description of the Related Art
A high-frequency signal measuring apparatus detector having a cabled detector, such as a network analyzer or a level measuring device, generally has the following unique problems. That is, since a cabled detector is naturally, easily affected by signal attenuation or external noise, a dynamic range is limited and the measuring result has a large amount of errors.
For this reason, a high-precision measuring technique applied to a scalar network analyzer for measuring a value of transmission or impedance of a circuit (circuit element) as a function of the frequency of an input signal is disclosed in U.S. Pat. No. 4,647,845. According to this technique, a cabled detector is arranged as an AC/DC detector used by switching two detector modes, i.e., a DC detector mode for a continuous wave input signal and an AC detector mode for a high-frequency pulse input signal. An output from the AC/DC detector is added to an AC network analyzer as a test signal through a cable line.
That is, according to the scalar network analyzer, on only an AC/DC detector side serving as a cabled detector, measuring error mixing caused by the signal attenuation or the external noise is prevented to realize high-precision measurement, and the scalar network analyzer has an arrangement shown in FIG. 1.
An AC/DC detector 18 includes an RF detector 40 that is connected to a chopper 44. The RF detector 40 detects an RF signal 28 supplied on a line 16 and transforms it into a rectified signal supplied on a line 42. The magnitude of the rectified signal supplied on the line 42 is proportional to the amplitude of the RF signal 28.
In the DC detection mode, the chopper 44 modulates the rectified signal supplied on the line 42 to produce a modulated signal supplied on a line 46. In the AC detection mode, the chopper 44 simply supplies the modulated signal, unaltered, on the line 42. In the AC detection mode, the chopper 44 does not modulate the rectified signal because when a pulsed RF signal is rectified, that rectified signal is already a modulated signal.
The modulated signal supplied on the line 46 is amplified by a preamplifier 48 to produce an amplified signal supplied on a line 50. In the DC detection mode, this amplified signal supplied on the line 50 is sampled by a sampler 52 to produce a sampled signal, supplied on a line 54. Sampling is done to eliminate any feed generated by the chopper 44. In the AC detection mode, no sampling is needed since no chipping has taken place. Here, the sampler 52 simply supplies the amplified signal on the line 54 as the sampled signal.
A buffer-compensator 56 buffers the sampled signal supplied on the line 54 and removes from the sampled signal supplied on the line 54 an unwanted signal due to a parasitic capacitance of the sampler 52. This produces modulated signal 30 supplied on a line 20.
An oscillator and timing logic 58 controls both the chopper 44 and the sampler 52. The chopper 44 is controlled by a first control signal supplied on a line 60 and a second control signal supplied on a line 62. The sampler 52 is controlled by a third control signal supplied on a line 64. As described in greater detail later, the first control signal and the second control signal open and close the chopper 44. The third control signal opens and closes the sampler 52.
The oscillator and timing logic 58 is controlled by a mode selector 59. The mode selector 59 establishes in which the mode AC/DC detector 18 will be set. While set in the AC detection mode, the timing and oscillator logic 58 provides the first and second control signals to the chopper 44 and the third control signal to the sampler 52 such that no chopping or sampling is done. While set in the DC detection mode, the timing and oscillator logic 58 provides the chopper 44 with the first and second control signals which open and close the chopper 44 at a frequency of 27.77 kHz, and the sampler 52 with the third control signal which opens and closes the sampler at a frequency of 55.55 kHz. Both modes supply the modulated signal 30 on the cable line 20.
In the above arrangement, since a preamplifier having a sufficiently small DC drift is used as the preamplifier 48, the circuit arrangement of the preamplifier is complicated and expensive. Therefore, the chopper 44 is provided to convert a DC signal into a pulse signal having a predetermined frequency and to suppress an influence of current level variation in the preamplifier 48.
The signal to be measured converted into a pulse signal input to an AC detecting network analyzer (measuring equipment body) 1 is amplified by a main amplifier 11 again, detected by a detector 12, and converted into a DC signal again. The signal to be measured converted into the DC signal is input to an analyzer 14 through a low-pass filter 13. The analyzer 14 performs predetermined analysis for the DC signal to be measured which is input, and the result is displayed on, e.g., a display screen of an operation panel (not shown).
As described above, since the signal to be measured input from the signal detecting terminal or connector 6 is detected and amplified by an AC/DC detector 18 and transmitted to the measuring equipment body 1 through the cable line 20, a decrease in S/N ratio due to attenuation of the signal to be measured or noise in the cable line 20 can be prevented compared with a case wherein the signal to be measured is input from the signal detecting terminal or connector 6 through the cable line 20 to the measuring equipment body 1.
In the above description, the preamplifier 48 is a simple linear amplifier, and a log amplifier is used as the main amplifier 11 to perform a dB display.
However, even in the scalar network analyzer shown in FIG. 1, the following problems are posed.
One of amplitude characteristics required by an amplifier incorporated in a measuring equipment is a linear amplitude characteristic in which a gain is constant independently of a signal level of an input signal. FIG. 2 is graph showing a relationship between an input signal level V.sub.I and an output signal level V.sub.O of the preamplifier 48 installed in the AC/DC detector 18. However, in the preamplifier 48 having the linear amplitude characteristics, a dynamic range is limited so that an excessive signal is input to saturate an output signal. A signal level of the input signal corresponding to a saturation level V.sub.OH of the output signal is a maximum input value V.sub.IH.
When the output signal level V.sub.O is excessively low, since an amplified signal to be measured is transmitted through the cable line 20 constituted by a cable line having a length of 1 to 2 m, external noise is mixed in the signal to be measured, thereby decreasing an S/N ratio when the signal is input to the measuring equipment body 1. Therefore, a minimum input value V.sub.IL is a signal level of an input signal corresponding to a lower limit level V.sub.OL which can assure an S/N ratio having a predetermined value or more.
Thus, since an allowable range of the input signal level V.sub.I to the preamplifier 48 is limited within the range of V.sub.IL to V.sub.IH an allowable measuring range of the signal to be measured of the whole scalar network analyzer is limited within the range of V.sub.IL to V.sub.IH.
When a signal to be measured has a signal level lower than the input minimum value V.sub.IL or when a signal to be measured has a signal level higher than the input maximum value V.sub.IH, measuring precision of the whole measuring equipment may be degraded.